Magnetically shielded probe card

ABSTRACT

A probe card includes a mechanical support fixture having an inner aperture with a plurality of probes secured to the fixture that includes probe tips that extend into the inner aperture for contacting probe pads on die of a wafer to be probed. At least one magnetic shield includes a magnetic material that at least substantially surrounds a projected volume over an area that encloses the probe tips. The magnetic material has a relative magnetic permeability of at least 800.

FIELD

Disclosed embodiments relate to probe cards for probing semiconductorwafers having integrated circuit (IC) die that include at least onemagnetically sensitive portion.

BACKGROUND

During a semiconductor fabrication process semiconductor die are formedon a wafer by processing including photolithography, deposition,implantation and etching. The wafer is a substrate generally formed of asemiconductor, such as silicon or gallium arsenide. After thefabrication process is completed and before the wafer is singulated intodie (or chips), the wafer has functional tests to verify theirelectrical performance within a design specification. Conventionally, atest head of a test apparatus for die probing usually mounts a probecard with a plurality of probe needles or other contact members forcontacting with probe pads (bond pads or bumps) on the die. The probecard provides electrical connections for interfacing between the testapparatus and the device (die) under test (DUT).

A probe card includes a probe card board (e.g. printed circuit board(PCB)) with a hollow center having a plurality of probe card needle tipsthat emerge from the center, extending downward, and are arranged tocontact the probe pads on the die to be probed. One probe arrangement isan epoxy card PCB with a ring assembly. The ring assembly is built byplacing preformed probes into a plastic template. Holes corresponding tothe pattern of the bond pads of the die to be tested are punched intothe template. A ceramic or anodized aluminum ring is epoxied to theprobes. The ring and epoxy are configured to hold the probes in theirproper orientation permanently.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed embodiments recognize some integrated circuit (IC) devices(such as magnetic “fluxgate” sensors) need ambient magnetic field levelsreduced to <50 μT to 100 μT to properly perform magnetically sensitiveelectrical measurements. One known arrangement for providing suchreduced ambient field levels assembles the sensor device in a package,and places the packaged sensor device along with its test board within alarge-scale concentric cylindrical magnetic shield (shield enclosure)typically capped at either one or both ends, where it is tested whilebeing within. The shield enclosure solution is recognized as being anexpensive arrangement (build cost plus package assembly cost) with lowtest throughput because typically a single packaged device is manuallyinserted into a test socket of the test board which is then insertedinto the shield enclosure. Additionally, the process of assembly of theIC device eliminates any possible wafer location information, whichmakes test correlation to wafer processing effects and sources of yieldloss difficult if not possible to be obtained.

Disclosed embodiments include magnetically shielded probe cards(shielded probe cards) that integrate a magnetic material (e.g.,MuMETAL® or similar magnetic material)) as magnetic shield layer(s) intothe design of the probe card itself, with an inner shield layerproximate to the probe pads of the device (die) under test (DUT).Disclosed shielded probe cards shield the DUT region from unintentionalambient magnetic fields, including earth's magnetic field and ambienttester fields, as well as attenuating magnetic fields generated bycurrent running through traces on the shielded probe card itself.

Disclosed shielded probe cards can comprise a mechanical support fixturehaving an inner aperture with a plurality of probes secured to thefixture that include probe tips that extend into the inner aperture forcontacting probe pads on die of a wafer to be probed. At least onemagnetic shield includes a magnetic material that at least substantiallysurrounds a projected volume projected 90 degrees above an area thatencloses the probe tips (see FIG. 2 described below). As used herein“substantially surrounding” refers to at least 80% of the length of thecorresponding fully enclosed shape such as the circumference (orperimeter) of a circular (ring-shaped) magnetic shield in one particularembodiment. The magnetic material has a relative magnetic permeabilityof at least 800.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIG. 1A is a top view of an example magnetically shielded probe cardincluding both an inner magnetic shield and an outer magnetic shield,according to an example embodiment.

FIG. 1B is a cross sectional view of the example shielded probe cardshown in FIG. 1A over a wafer that is on chuck that also has a disclosedmagnetic shield, according to an example embodiment.

FIG. 2 shows an example shielded probe card including both an inner andan outer magnetic shield embodied as an epoxy card with ring assembly,according to an example embodiment.

FIG. 3 depicts an example probe system including a disclosed shieldedprobe card shown as the shielded probe card in FIG. 2, according to anexample embodiment.

FIG. 4 shows the magnetic field strength at the z-height of the deviceunder test (DUT) as a function of the radial distance from the DUT whenprobing a wafer using a disclosed shielded probe card, according to anexample embodiment.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

Also, the terms “coupled to” or “couples with” (and the like) as usedherein without further qualification are intended to describe either anindirect or direct electrical connection. Thus, if a first device“couples” to a second device, that connection can be through a directelectrical connection where there are only parasitics in the pathway, orthrough an indirect electrical connection via intervening itemsincluding other devices and connections. For indirect coupling, theintervening item generally does not modify the information of a signalbut may adjust its current level, voltage level, and/or power level.

FIG. 1A is a top view of an example shielded probe card 100 includingboth an inner magnetic shield 120 and an outer magnetic shield 130,according to an example embodiment. Shielded probe card 100 comprises amechanical support fixture comprising a dielectric or adielectric/conductor fixture (fixture) 105 with an inner assembly 106 tosecure the probe tips 103 a of a plurality of probes 103 (e.g., tungstenprobes). For example, shielded probe card 100 can comprise a ceramicfixture having an inner aperture (or cavity) 102 with the probes 103embedded within the inner assembly 106 of the fixture 105. The probetips 103 a extend into the inner aperture 102 for contacting probe pads(bond pads or solder bumps) on die of a wafer to be probed. The unshadedregion depicted between the fixture 105 and outer magnetic shield 130represents a dielectric that is included when the fixture 105 does notcomprise a dielectric material (e.g. comprises metals such as aluminumor stainless steel). This dielectric is not necessarily solid orcontinuous, and can comprise a silicone film, ceramic, etc.

The inner magnetic shield 120 and outer magnetic shield 130 bothcomprise a magnetic material that at least substantially encloses aprojected volume (see projected volume 240 in FIG. 2 described below)over an area that encloses the probe tips 103 a by completelysurrounding the projected volume, shown in a concentric arrangement(having a common center) providing a continuous ring. However, acontinuous magnetic shield ring is not a requirement, although theattenuation performance of the shielded probe card generally dropssignificantly with the size of any gap or slotting in the magneticshield. Some limited degree of gap or notching/slotting along the lengthof the magnetic shield may be generated during mechanical assembly.

The thickness of the magnetic material is generally between 0.05 mm and3 mm, and is typically 0.5 mm to 3 mm thick. The magnetic material canbe provided in a foil configured in a shielded can form (e.g., eitherround or rectangular shapes) for the inner magnetic shield 120 or insheet form to provide the desired area coverage for the outer magneticshield 130. For example, MuMETAL® is commercially alloy available fromMagnetic Shield Corporation Bensenville, Ill., in a variety of forms,including shielded can and sheet form in gauges from 0.36 mm to 1.57 mm.

Given probe card embodiment as shown in FIG. 1, inner magnetic shield120 can be physically mounted to the fixture 105 by various ways,including a pressure fit assembly to either fixture 105, to pressure-fitsupports built into fixture 105, or through application of adhesive.Outer magnetic shield 130 can be physically attached to the outside offixture 105 through a similar attachment.

As noted above, the magnetic material has a relative magneticpermeability of at least 800. In some embodiments the relative magneticpermeability of the magnetic material is at least 5,000, such asMuMETAL® which comprises a nickel-iron soft magnetic alloy, about 50% to80% nickel, that has a relative magnetic permeability ranging from about5,000 to about 400,000. In other embodiments the magnetic permeabilityis between 800 and 5,000, such as comprising an ultra-low carbon steelshielding material that has quoted magnetic permeability values of˜1,000 marketed as AMUNEAL®.

The inner magnetic shield 120 can comprise a hollow sleeve positionedwithin the inner aperture 102 that is secured to the fixture 105, suchas by a pressure fit. The inner magnetic shield 120 can have an outerdiameter of 3 mm to 8 mm. The outer magnetic shield 130 is physicallymounted onto a surface of the fixture 105, or is supported by somesurface of the shielded probe card 100. It may also be possible to embedmagnetic material for the outer magnetic shield 130 to be within thefixture 105 through lamination.

FIG. 1B is a cross sectional view of the example shielded probe card 100shown in FIG. 1A over a wafer 190 that is on a wafer chuck 170 that alsohas a disclosed magnetic shield 160 shown below the wafer 190, accordingto an example embodiment. The magnetic shield 160 can comprise a flatdisc of magnetic material (solid or perforated) of sufficient magneticpermeability. The magnetic shield 160 can also be coated with adielectric material (e.g. polytetrafluoroethylene (PTFE, trade nameTEFLON) to electrically isolate it from the wafer and/or wafer chuck(see dielectric layer 362 shown in FIG. 3 described below). The magneticshield 160 can be attached in a variety of ways including mechanicalclamping, by a chuck vacuum with a shield vacuum hole design allowingsome thru-holes to enable a vacuum to hold the wafer on top of a shielddisc, and adhesion.

There can be more than two concentric magnetic shield layers if there isenough room within the inner aperture 102 and the shield material isthin enough. For cases where there are multiple shield layers (≥2), theinnermost magnetic shield layers can be physically attached to the outermagnetic shield layers only (e.g., using a dielectric spacer material,such as TEFLON, epoxy, silicone, etc.), without the need to physicallymount them to any probe card surface. Only one of the shield layers inthis embodiment is generally mounted to a surface of the probe card.

FIG. 2 shows an example shielded probe card 200 including both an innermagnetic shield 120 and an outer magnetic shield 130, embodied as a PCBprobe card with epoxy ring assembly comprising a PCB 210 and ringassembly 220, according to an example embodiment. The ring assembly 220fits in the PCB aperture 210 a, and the ring assembly 220 has its ownring aperture 220 a that defines the inner aperture 102. Probes 103 aresecured to the ring assembly 220 by epoxy 225. A projected volume overan area that encloses the probe tips 103 a of the probes 103 is shown as240. The inner magnetic shield 120 and an outer magnetic shield 130 areboth shown to enclose the projected volume 240, both being concentricwith the projected volume 240.

Inner magnetic shield 120 should generally be as close as possible, orcoincident with the projected volume 240 to maximize the magneticattenuation provided. The z-height of inner magnetic shield 120 shouldgenerally be extended down to be as close as possible to the probe tips103 a (without ever touching) to minimize the gap between inner magneticshield 120 and the DUT and extended up as the design dictates tomaximize magnetic attenuation while still fitting within the confines ofthe probe system. An example range for the z-height (perpendicular tothe plane of the wafer) of the inner magnetic shield 120 above the wafer190 is from 150 μm to 1.5 mm, dependent at least in part on the depth ofthe probe tips 103 a, the z-thickness of the PCB 210, and the probe carddesign. Outer magnetic shield 130 should generally similarly be extendedbeyond the z-thickness of the PCB 210.

For probe card configurations with sufficiently small inner apertures102 with sufficient probe length underneath the fixture material, theinner magnetic shield 120 can be a shield layer that sits on top of thefixture material (e.g., on ring assembly 220), effectively enclosing theinner aperture 102. This arrangement increases the distance of the innermagnetic shield to the DUT, but this design can potentially compensatefor the associated decrease in the attenuation rating (or might not,depending on the ambient field requirements of the test).

A disclosed shielded probe card has several advantages compared to aknown shielded probe system. A shielded probe card can be used onmultiple different probe systems by simply relocating the probe card.Additionally the probe card design can be modified for similarlydesigned probe cards. This is a significantly less expensive solutionthan either purchasing an enclosure that fully shields the probe systemor retrofitting an existing probe system with such a shielded enclosure.

A disclosed shielded probe card also places the magnetic shield as closeas is possible to the DUT, which isolates the DUT from almost allexternal fields. A conventional outer shielded enclosure in contraststill leaves the DUT subject to fields generated by the probe card andcabling inside of the enclosure. A disclosed shielded probe card may beparticularly important for wafer-level test systems that include anoptics port over the DUT, which cannot be blocked by a conventionalshielded enclosure.

FIG. 3 depicts an example probe system 300 including a wafer prober 320including a disclosed magnetically shielded probe card shown as theshielded probe card 200 in FIG. 2, according to an example embodiment.Shielded probe card 200 is shown including a test (or probe) head 302 ona performance board 304. In a typical parametric probe system, the testhead 302 docks directly to the probe card so that no performance board304 is needed. Disclosed shielded probe card 200 can be applied toeither a product multi-probe system or to a device parametric testsystem.

For system 300 signals are received by the performance board 304 from atest controller such as automatic test equipment (ATE) 310 via leadlines 312 which may include digital, high frequency, high precisionanalog, RF and/or power paths. In a parametric probe system (without aperformance board 304), signals are routed directly from the test head302 without need for lead lines 312. Probe card 200 has contact pointsprovided by probe tips 103 a of the probes 103 in a specific array tomirror the corresponding contact points of a specific design of the DUTson the wafer 190. The wafer 190 is on an optional dielectric layer 362such a TEFLON sheet or coating on a magnetic shield layer 160 (e.g.,MuMETAL®) that is on a wafer chuck 170 that is on an X, Y, Z, θ stage175. The probes 103 may be conventionally soldered to the PCB 210. Thewafer chuck 170, dielectric layer 362 and magnetic shield layer 160 maybe patterned to allow a vacuum to be pulled on the backside of the wafer190 from below.

The probe system 300 is also shown including a computer unit 315 forcontrolling the ATE 310 and the test control unit 311. A parametricprobe system can be operated without a controlling PC such as computerunit 315, only needing parametric test instrumentation and manual X, Y,Z, θ control of the wafer chuck 160. The test controls signals and testdata is delivered to and from the ATE 310 and DUTs on the wafer 190through the lead lines 312 and the shielded probe card 200.

FIG. 4 shows the magnitude of the magnetic field at the z-height of theDUT as a function of the radial distance parallel to the wafer surfacewhen probing the wafer using a disclosed magnetically shielded probecard, with an external B-field (Bext)=50 μT, according to an exampleembodiment. Simulation data is shown for combinations of the innermagnetic shield 120 outer diameter and shield layer thicknesses. AB-field of <500 nT (<1% of Bext) is shown reaching the DUT surface. Thishigh level attenuation of Bext provided by the shielded probe cardenables measurement of extremely sensitive magnetic components at waferlevel, substantially improving throughput and ability to correlatedevice metrics to wafer level processes. It also decreases existinglearning cycles, since package assembly time is no longer needed toacquire data, and enables complete characterization of process splits.

Disclosed embodiments can be used to test a variety of different ICdevices and related products. The IC die on the wafers may includevarious elements therein and/or layers thereon, including barrierlayers, dielectric layers, magnetic layers, device structures, activeelements and passive elements including source regions, drain regions,bit lines, bases, emitters, collectors, conductive lines, conductivevias, etc. Moreover, the IC die can be formed from a variety ofprocesses including bipolar, CMOS, BiCMOS and MEMS.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure.

The invention claimed is:
 1. A probe card, comprising: a mechanicalsupport fixture having an inner aperture with a plurality of probessecured to said fixture that include probe tips that extend into saidinner aperture for contacting probe pads on die of a wafer to be probed,and at least one magnetic shield comprising a magnetic material that atleast substantially surrounds a projected volume over an area thatencloses said probe tips of said probes, wherein said magnetic materialhas a relative magnetic permeability between 800 and 5,000.
 2. The probecard of claim 1, wherein said magnetic shield comprises a hollow sleevepositioned within said inner aperture that is secured to said fixture.3. The probe card of claim 1, wherein said magnetic shield is physicallymounted onto a surface of said fixture.
 4. The probe card of claim 1,wherein said magnetic shield comprises a first magnetic shield includinga hollow sleeve positioned inside said inner aperture that is secured tosaid fixture and a second magnetic shield that is physically mountedonto a surface of said fixture.
 5. The probe card of claim 4, whereinsaid second magnetic shield concentrically surrounds said first magneticshield.
 6. The probe card of claim 1, wherein a thickness of saidmagnetic material is between 0.05 and 3 mm.
 7. The probe card of claim1, wherein said magnetic shield completely surrounds said projectedvolume.
 8. A wafer probe system, comprising: a wafer prober including atest head coupled to a probe card for probing die of a wafer that isdisposed on a wafer chuck; wherein said probe card comprises: amechanical support fixture having an inner aperture with a plurality ofprobes secured to said fixture that include probe tips that extend intosaid inner aperture for contacting probe pads on die of a wafer to beprobed, and at least one magnetic shield comprising a magnetic materialthat at least substantially surrounds a projected volume over an areathat encloses said probe tips of said probes, wherein said magneticmaterial has a relative magnetic permeability between 800 and 5,000. 9.The probe system of claim 8, wherein said wafer chuck includes amagnetic shield comprising a magnetic material that at leastsubstantially surrounds a projected volume under said area that enclosessaid probe tips.
 10. The probe system of claim 9, further comprising amagnetic shield on a dielectric layer on said wafer chuck.
 11. The probesystem of claim 8, wherein said magnetic shield comprises a hollowsleeve positioned within said inner aperture that is secured to saidfixture.
 12. The probe system of claim 8, wherein said magnetic shieldis physically mounted onto a surface of said fixture.
 13. The probesystem of claim 8, wherein said magnetic shield comprises a firstmagnetic shield including a hollow sleeve positioned inside said inneraperture that is secured to said fixture and a second magnetic shieldthat is physically mounted onto a surface of said fixture.
 14. The probesystem of claim 13, wherein said second magnetic shield concentricallysurrounds said first magnetic shield.
 15. The probe system of claim 8,wherein a thickness of said magnetic material is between 0.05 mm and 3mm.
 16. The probe system of claim 8, wherein said magnetic shieldcompletely surrounds said projected volume.